Microplastic quantification in wastewater

During passage through a wastewater treatment plant (WWTP), a plastic particle may be removed from wastewater via screening or incorporated into biosolids or discharged to the environment in treated effluent.

The biosolids produced during wastewater treatment are often used as a fertiliser. 
The risks posed by microplastics entering a wastewater treatment plant is dependent on where they are captured or released into the environment. Unfortunately, detecting microplastics in waste is a challenging task, as different test methods capture different particle size ranges and assign polymer types with varying levels of confidence.

To assess microplastics removal from water during treatment, measurements were taken at multiple points along the wastewater treatment process, allowing us to understand how different sewage treatment processes can affect the number of microplastics entering waterways or soil.

Samples were collected from 2 wastewater treatment plants:

• one with primary-level treatment that screened the wastewater before sedimentation
• one with tertiary-level treatment that included additional processes such as activated sludge clarification and disinfection.

Samples were taken from the incoming wastewater, after screening, as well as from the outgoing treated water. To maximise relatedness, samples were taken from the same packet of wastewater entering and leaving the treatment plant. The difference in microplastic loads between the 2 was attributed to the incorporation of microplastics into solid wastes like biosolids. 

Sampling of biosolids was also performed at 7 additional sites since biosolids were identified as carrying the greatest loads of microplastics that enter the WWTPs. Measurements were taken using Fourier-transformed infrared (FTIR) spectroscopy, a form of visual microscopy that can determine the size, polymer type, and number of microplastics present. 

The concentration of microplastics in the water decreased significantly from the water's entry point to its discharge point in the environment. Concentrations of microplastics in the water decreased by 79% in the primary STP and over 98% in the tertiary STP. Microplastic concentrations remained relatively constant before and after screening, suggesting that most microplastics entering a wastewater treatment plant likely end up in the biosolids. 

In both plants, polypropylene (PP) was identified as the most common microplastic polymer, representing between 63% and 87% of the total microplastic load in each sample, with polyethylene terephthalate (PET) and polyethylene (PE) accounting for the bulk of the non-PP materials. Microplastic fragments were the most common morphology identified, followed by textile microfibers, and then microbeads, which were the least common. 

Examination of the biosolids themselves revealed similar trends, with the PP being the most commonly detected microplastic type across the 7 treatment plants.

It’s likely that the greater the treatment level of a WWTP, the fewer microplastics will be directly discharged into the environment via wastewater.

However, the majority of microplastics entering wastewater treatment plants are likely to be captured and potentially applied to land within biosolids. 

The NSW EPA is currently reforming the management of biosolids within NSW, with this work contributing to our understanding of the risk posed by microplastics. The EPA is working to refine microplastic test methods, aiming to develop an accurate, scalable, and cost-effective method for wider adoption within regulatory frameworks.